1. Field of the Invention:
The present invention relates to resin molded products which are designed to display extinction of activity.
2. Description of the Prior Art:
There have been known in the art so-called controlled-release products which are adopted to slowly release active agents, (e.g., fungicides, pesticides, etc.) incorporated into inert resin molded products. With such controlled-release products, as might be expected from their retention of activity for a prolonged period, eventually the time during which the active agents are released effectively lapses and the rate of release of the active agents becomes too low to recognize the extinction of activity. Further, since the rate of release of active agents from controlled-release products may vary depending on temperature, humidity and other conditions, their service lives are not always constant and the effective period indicated on the product label can only supply a very rough estimate. Accordingly, it is often the case with such products that exhausted materials are falsely thought to be active or materials still retaining satisfactory activity are thought useless and discarded.
U.S. Pat. No. 4,062,649 discloses a method for indicating depletion of controlled-release pesticide formulations which relies upon a color change in an acid-base indicator. The method, however, requires the incorporation of no less than two additional reagents in the pesticide formulation and, as such, it is not cost effective. In the case of oxygen absorbents, a product is known (Ageless by Mitsubishi Gas Chemical Co.) which is designed to display the exhaustion of activity by incorporating a reagent which will react with oxygen and change color when the active agent has been depleted and oxygen has resumed its presence in the atmosphere. However, the art holds little information on controlled-release products in which the resin molded product itself is designed to display extinction.